1. Field of the Invention
This invention relates to an optical memory medium with which information can be optically recorded and reproduced or erased. More particularly, it relates to an improved preformat of an optical memory medium.
2. Description of the Prior Art
In recent years, with the development of information processing systems, inexpensive large capacity memories have been required. To meet this requirement, various optical memory systems have been developed. Such optical memory systems are roughly classified into three categories: read-only type; write once type; and rewritable type.
In a write once or rewritable optical memory medium information is recorded, reproduced or erased by irradiating a light beam such as a laser beam along tracks (hereinafter, referred to as "recording tracks"). In order to accurately guide a laser spot to an objective position of the optical disk, grooves for guiding the laser spot (hereinafter, referred to as "guide grooves") and pits for prerecorded information are preformed on a glass or plastic substrate. Such pits are called "prepits". The information recorded in the prepits includes generally the track address of a recording track and synchronizing signals for detecting said information. When a recording track is divided into sectors, the prerecorded information also includes the sector address.
An example of the preformat portion in a prior art optical memory medium is diagrammatically shown in FIG. 11A. In the exemplified configuration, guide grooves 1 are formed concentrically or spirally on the surface of a substrate 2 and the information is recorded on said guide grooves 1. Prepits 3 having a variety of lengths are formed in alignment with the guide grooves 1. Then, a recording layer made of a suitable material is formed by a vacuum deposition, spattering method or spin coating method. Then, the substrate is covered by a protective substrate, or applied with a hard coating composition is applied, to obtain an optical memory medium.
When the guide grooves 1 and prepits 3 are scanned by a laser spot, signals having the waveform as shown in FIG. 11B are obtained from the reflected light beam. The obtained signals are modulated in accordance with the lengths of the prepits 3. The signals will be demodulated and binary coded by setting an appropriate slice level. It is known that the modulated signals are closely related to the physical structure of the prepits 3. FIGS. 6A and 6B diagrammatically illustrate that the reflected light beam from the optical recording medium is vignetted in an object lens of a reading/recording apparatus, due to the diffraction effect in the pit portions and the unpitted portions. As seen from FIGS. 12A and 12B, there exists a pattern difference of the intensity between the light beam reflected from an unpitted portion (FIG. 12A) and that reflected from a pitted portion (FIG. 12B). Hence, the strength distribution of the reflected light beam is changed to produce the variation in the light intensity passing through the object lens.
FIG. 13 shows the relation between the pit widths .tau. which are normalized by the diameter of the light spot (P.times.1/e.sup.2 ; the diameter at which the light strength is 1/e.sup.2 times of the peak strength) and the signal levels obtained from prepits which are longer than the diameter of the light spot, using the depth .delta. normalized to the wavelength of the light spot as a parameter. In FIG. 13, the signal levels of the reflected beams are plotted in relation to the signal level obtained from unpitted portions. From FIG. 13, it will be seen that the degree of the deflection or the modulation of the reflected light beam is greatest when the width of the prepits is approximately 0.32 times of the diameter of the light spot. Therefore, the prepits are formed so that their width becomes as close to this value as possible.
Considering these properties, a substrate for an optical memory medium is produced as follows. A photoresist is applied to a glass substrate, and the substrate is exposed by scanning a laser beam of a short wavelength such as an Ar laser which is modulated in accordance with a coded signal of the information to be prerecorded. The portions exposed to the laser beam are removed by developing, to form prepits or grooves. Using the obtained original glass substrate as a master, the pattern of the original substrate is transferred to a glass or plastic substrate, to obtain a substrate for an optical memory medium. In a section C corresponding to a long prepit, the coded signal has square pulses produced continuously (FIG. 14).
In the above-described process of producing the original substrate, it often happens that uneven exposure, developing time and transfer cause a fluctuation in the width of the prepits or grooves of an optical recording medium, resulting in the disturbance of the signal reproduced from the prepits.
For example, the waveform obtained from long prepits 3' having a large width (FIG. 15A) is markedly distorted as shown in FIG. 15B. This possibly causes the erroneous detection of the edges of the pulses when binary-coding the data, resulting in misreading of address information or the like. The distorted waveform is likely to occur in the mid portion of a prepit where the width is large, rather than in the edge portions where the width is gradually increased or decreased. This is because the diffraction efficiency is lowered in the mid portion, and this appears remarkably when scanning a long prepit.
As described above, the control of the width of a prepit which is longer than the diameter of a light spot in producing an original substrate has a great influence on the restraint of the distortion of the reproduced signal in an optical memory medium. Hence, the structure of a conventional optical memory medium requires the fine control of the exposure conditions in producing the original substrate and also a precise control of accuracy in the transfer process, providing an obstacle to the improvement of the yield.
Another example of the preformat portion in a prior art optical memory medium is shown in FIG. 16. In the optical memory medium of FIG. 16, guide grooves 1 are formed on a substrate 4, and a record track 2 is formed between guide grooves 1 adjacent to each other. The information such as a track address is prerecorded in the form of prepits 3 formed on the record track 2. The width and depth of the guide grooves 1 have a large effect on the track count signal property for a high speed access operation, and on the track servo property for retaining a light spot on the record track 2. Consequently, the width of the guide grooves 1 is set within the range from about 0.3 .mu.m to about 0.5 .mu.m.
The prerecorded information is read out using the diffraction in the prepits 3. When a light spot moves from a to c as shown in FIG. 17A, the reflected light's intensity changes as shown in FIG. 17B. In FIG. 17B, I.sub.1 is the reflected light intensity obtained when the light spot is on the prepits 3, and I.sub.2 is the reflected light intensity obtained when the light spot is on a portion other than the prepits 3. The information is read out based on the difference between the light intensities I.sub.1 and I.sub.2. Therefore, the quality of the reproduced signal depends on the degree of the difference between the light intensities I.sub.1 and I.sub.2.
Because, as described above, the information is prerecorded in the form of prepits 3 formed on the record track 2 disposed between two guide grooves 1, the properties of the information signal which is read out from prepits 3 are greatly affected by the width and/or depth of the guide grooves 1. When the width of the guide grooves 1 is set to be 0.3 .mu.m to 0.5 .mu.m as stated above, the diffraction efficiency in the prepits 3 is lowered so that the difference between the light intensities becomes small, resulting in that a reproduced signal of high quality cannot be obtained from the prepits 3.